1. Field of the Invention
The present invention relates generally to a flow control device having a sleeve valve arrangement operable by means of a shifter tool. The invention has particular application for acidizing stimulation tools used in horizontal or other deviated wells.
2. Description of Related Art
Acidizing is a popular method of well stimulation in which the porosity and/or permeability of the reservoir is increased by pumping acid into portions of a formation adjacent to a well bore. In some wells, more than one zone will have been perforated or exposed by an open hole completion. Because the zones will often have different permeabilities, acid would tend to enter the zone with the highest permeability first. However, this zone would be the one least needing acid. A number of mechanical methods are known for selective placement of acid such that the majority of acid is placed into the lower-permeability zones. These methods include the use of ball sealers which are dropped into the well to block the perforations along a zone. In addition, packing elements and plugs may be used as seals to separate zones in a formation. Unfortunately, such elastomeric members may be unreliable, particularly at great well depths where portions of the tubing string may be maintained at temperatures at or above 200.degree. F. Ambient temperature acid pumped down into the well will cause the metallic elements of the tubing string to contract when the acid reaches these depths. As continued pumping of acid causes the formation to deteriorate and the rate of acid flow to increase, the contraction can be significant and cause loss of elastomeric seals with the seal bores.
A sleeve valve or sliding side door arrangement is an effective alternative to a elastomeric seal. Use of sleeve valves for selectively opening or closing a port is well known in the art of well drilling. Shifter tool arrangements that operate the sleeve valves are known which engage compatible profiled grooving in a well conduit. Such arrangements selectively locate and lock a shifter tool into compatible profiled grooving in a well conduit using upper and lower expander surfaces provided on an inner mandrel and which are moveable downwardly inside a set of keys for locking the keys in an expanded and engaged position. This action connects the shifter tool and the profiled grooving together until disconnect is desired. The shifter tool may be operated to retract the keys when required for disconnect from the profiled grooving by use of a hydraulic jar or methods such as the shearing of a shear pin or use of a ratchet type indexing system.
While arrangements such as these are useful for securing a shifter tool at a selected location within a well conduit, they have problems related to the release of the shifter tool from the conduit. Shear pin arrangements cannot be reset or reused without withdrawing the tool from the conduit to replace the shear pins. Jar and ratchet techniques involve significant manipulation of the tubing string from the surface and are time consuming.
Deviated wells, particularly horizontal wells, magnify operational problems associated with tool operation. These problems are significant for acid stimulation tools which incorporate sleeve valve arrangements due to the caustic, corrosive nature of acid. First, it is difficult to hydraulically balance the mud in a horizontal well. In a vertical well, mud provides a head to balance the downhole pressure from the well. In a horizontal well, however, there is no fluid head to balance the downhole pressure. If acid has been improperly placed within the well, such as into a portion with no access to the formation perforations, the acid may flow back out of the well. When shifter tools are removed from a horizontal or deviated well, the tools tend to swab out the mud along with any residual acid. Because the production zone may be 5000 feet or so out into a horizontal borehole, it is important that the operator know exactly the location of the stimulation tool within the tubing string.
Also, gravity disposed wireline devices cannot be used in these wells. Coiled tubing must be used to place the shifter tool properly for operation of the sleeve valves. Coiled tubing, however, will not structurally support the application of great compressive forces which are often used to "locate" the shifter arrangement within the housing such that acid may be safely flowed into surrounding perforations. The tubing can, however, support much greater loads in tension than in compression.
Horizontal wellbore location systems are known which employ an inner locator tool which is disposed within a ported housing. The tool is located within the housing under compression. As noted, however, compressive limitations exist for coiled tubing. It is difficult to accurately locate the stimulation tools such that the acid flow ports are located within the desired stimulation zone due to the tendency of the tool's components to "stack-up" as they are pushed into the wellbore. Because of the problems of hydraulically balancing horizontal wells. improperly placed acid may flow back out of a horizontal wellhead or be backwashed out when the tool is withdrawn. Therefore, it is desirable to develop a system for acidizing formations which avoids the problems of lost wellbore seals and permits more reliable location of stimulation tools within horizontal wellbores.